1. Field of the Invention
The present invention relates to improvements in methods and apparatus of the type commonly used to automatically and rapidly sort minute particles, e.g. biological cells, entrained in a moving liquid on the basis of certain predetermined particle characteristics. More particularly, it relates to a particle-sorting method and apparatus in which particles of interest are selectively extracted from their entraining liquid by using a hydraulic impulse to selectively eject such particles of interest from the entraining liquid.
2. The Related Prior Art
Flow cytometry is commonly used to differentiate various types of cells and other “formed bodies” comprising a biological fluid, e.g., whole blood. Conventional flow cytometers commonly comprise an optically-transparent flow cell, usually made of quartz, having a central channel through which a stream of cells to be individually identified is made to flow. Movement of the cell stream through the flow cell channel is hydrodynamically entrained to the central longitudinal axis of the flow cell channel by a cell-free sheath liquid that concentrically surrounds the cell stream and flows along with the cell stream as it passes through the flow cell channel. As each cell passes through a cell-interrogation zone of the flow cell channel, it is irradiated with a focused beam of radiation (as commonly provided by a laser source). Upon impinging upon each cell, the laser beam is scattered in a pattern characteristic of the morphology, density, refractive index and size of the cell. Further, the spectral characteristics of the laser beam may act to excite certain fluorochromes associated with selected cells, as may be the case when a cell's DNA has been previously stained with such fluorochromes, or when a fluorochrome molecule has been previously conjugated with a selected type of cell, either directly or via an intermediate bead or the like. Photodetectors strategically positioned about the optical flow cell serve to convert the light-scattered by each cell and the fluorescence emitted by the excited fluorochromes to electrical signals which, when suitably processed, serve to identify the irradiated cell. In addition to the light scatter and fluorescence measurements made on each cell, some flow cytometers further characterize each cell by measuring certain physical and/or electrical properties of each cell as it passes through the flow cell. Using the well-known Coulter Principle, a DC and/or an RF current is caused to pass through a constricted aperture in the flow cell channel simultaneously with the movement of cells therethrough. The volume of each cell affects the level of DC current through the flow cell aperture, and the cell's electrical conductivity affects the RF current through such aperture. See, for example, the flow cytometer disclosed in the commonly assigned U.S. Pat. No. 6,228,652, issued in the names of Carlos M. Rodriguez et al.
A conventional light scatter and fluorescence-sensing flow cytometer of the type noted above is disclosed in U.S. Pat. No. 3,710,933 issued to Mack J. Fulwyler et al. To this standard flow cytometer, Fulwyler et al. have added a cell-sorting component that operates to selectively remove and collect certain cells of interest (e.g., abnormal cells) from the effluent of cells that have already passed through the optical flow cell and have been identified as to cell type. More specifically, the cell-sorting component comprises a piezoelectric device that acts to vibrate the flow cell so as to effect the production a stream of droplets from the cell-entraining sheath liquid exiting from the flow cell. Ideally, each droplet contains but a single cell that has been characterized as to cell type by the light-scatter and fluorescence measurements just made on such cell. Each droplet in the droplet stream is then electrostatically charged as it passes between a pair of electrically charged plates, and each charged droplet is selectively deflected (or not deflected) towards a collection container as it passes between a pair of electrostatically charged deflection plates, such plates being charged to a droplet-deflecting polarity only at a time to deflect droplets (and cells) of interest. The instantaneous polarity of the deflection plates is determined by a cell-characterization processor that processes the cell-measurement signals from the optical flow cell.
In cell-sorting flow cytometers of the above type, the continuous production of suitably sized droplets can be problematic. Not only is it technically difficult to continuously produce droplets that contain only a single cell, but also the required size of the droplets is so small (aerosol in size) that it is difficult to control their precise movement as they exit from the flow cell. Typically, when it is suspected that a droplet contains more than one cell, the droplet is allowed to proceed to a waste container in order to avoid potential contamination of the collected cells of interest with other cells.